Elsevier

Catalysis Today

Volume 65, Issues 2–4, 20 February 2001, Pages 163-169
Catalysis Today

The influence of the chlorine content on the bimetallic particle formation in Pt–Re/Al2O3 studied by STEM/EDX, TPR, H2 chemisorption and model reaction studies

https://doi.org/10.1016/S0920-5861(00)00561-7Get rights and content

Abstract

A Pt–Re/Al2O3 reforming catalyst with different levels of chlorine content prior to reduction has been studied by various techniques such as combined STEM/EDX, TPR, H2 chemisorption and model reactions in order to investigate the effect of the chlorine content on the bimetallic particle formation. TPR, H2 chemisorption and model reactions show that chlorine inhibits the formation of bimetallic particles in the Pt–Re/Al2O3 catalyst. The effect of chlorine is, however, limited. Direct measurements by STEM/EDX analysis could not reveal any significant differences in alloy formation by varying the chlorine content from 0.6 to 1.5 wt.%. In comparison, the effect of adding water during reduction has a greater impact on the final state of the metal particles.

Introduction

Since the introduction of the Pt–Re(S)/γ–Al2O3–Cl catalyst in catalytic reforming [1], this system has been extensively studied with regard to the role of rhenium. This is a bifunctional catalyst where platinum is the active hydrogenation/dehydrogenation catalyst, and the acidic support catalyses the isomerisation and cracking reactions. The acidity is adjusted by adding chlorine. Over time the bimetallic catalyst loses its activity especially due to coking. The addition of rhenium to the monometallic Pt/Al2O3–Cl catalyst increases its stability, but the way that rhenium modifies this system is not clear. It seems to be a general agreement [2], [3], [4], [5], [6] that rhenium has to be present “alloyed” with platinum in small bimetallic particles in order to have a beneficial effect. In industrial units sulphur is added to the catalyst in order to suppress undesired reactions. After a certain time on stream, the catalyst has to be regenerated by burning off the coke. Under these conditions platinum will sinter, and in order to re-establish the platinum dispersion and to restore the acidity of the support, the catalyst undergoes oxychlorination treatment followed by reduction. It has been reported that the chlorine level prior to reduction is critical for the alloy formation, but the conclusions are diverse. Michel et al. [7] suggest that there is a mobile rhenium oxychloride involved in the reduction mechanism, whereas others [8], [9] claim that chlorine inhibits Pt–Re interaction. We have previously studied the formation of bimetallic particles on the Pt–Re/Al2O3 catalyst employing STEM/EDX, TPR, H2 chemisorption and model reactions with respect to the pre-treatment conditions [10], [11]. Evidence for alloy formation was found and it was shown that alloy formation was influenced by the drying temperature prior to reduction. The present study is aimed at investigating the effect of the chlorine content on the bimetallic particle formation in the same catalyst system.

Section snippets

Catalysts

The catalyst used was EUROPT-4 (Akzo) 0.3–0.3 wt.% Pt–Re/γ–Al2O3. As received it contained approximately 1.0 wt.% Cl. To adjust the chlorine level, the catalyst was subjected to oxychlorination treatment at 500°C by using a flow of air saturated with different ratios HCl/H2O. Finally, the catalyst was dried in air at 510°C. The chlorine content of these samples was measured to be 0.6, 1.1, and 1.5 wt.%, respectively, by Escha dissolution and silver nitrate titration using chromate as indicator

Model reactions and hydrogen chemisorption

The results from the hydrogenolysis of n-butane performed on samples 1, 2, and 3 are presented in Table 1, and the data given is from the first pulse. The selectivity to methane is taken as a measure of the degree of Pt–Re interaction, and it can be observed that the sample with the highest chlorine content show the lowest activity for n-butane conversion and selectivity to methane. Sample 2 shows a slightly higher selectivity to methane than sample 1, but the difference is hardly significant.

Conclusions

Using different experimental techniques, the effect of the chlorine content prior to reduction on the bimetallic formation in the Pt–Re/Al2O3 catalyst has been studied. Indirect methods such as model reactions, hydrogen chemisorption, and TPR studies indicate that too much chlorine on the catalyst surface during reduction is undesirable with respect to bimetallic particle formation. Chlorine has apparently an inhibiting effect on the mobility of Re in the reduction process. The effect of

Acknowledgements

The financial support of the Norwegian Scientific Foundation through the VISTA program is gratefully acknowledged.

References (18)

  • C. Betizeau et al.

    J. Catal.

    (1976)
  • P. Biloen et al.

    J. Catal.

    (1980)
  • V.K. Shum et al.

    J. Catal.

    (1985)
  • S.M. Augustine et al.

    J. Catal.

    (1989)
  • C.G. Michel et al.

    Fuel Process. Technol.

    (1993)
  • P. Malet et al.

    J. Catal.

    (1989)
  • C.L. Pieck et al.

    Appl. Catal.

    (1995)
  • R. Prestvik et al.

    J. Catal.

    (1998)
  • R. Prestvik et al.

    J. Catal.

    (1998)
There are more references available in the full text version of this article.

Cited by (21)

  • Promoted Re/Al<inf>2</inf>O<inf>3</inf> systems as sour water-gas shift catalysts

    2020, Catalysis Today
    Citation Excerpt :

    It is being investigated for many industrial chemical processes and presents the subject of scientific investigation for many years. For decades, one of its main applications is in hydrogen generation industry as promoter in reforming Pt-Re bifunctional catalysts [17–22]. Another known bimetallic system is alumina-supported Ni-Re catalyst for steam reforming of gasoline fuels.

  • Effect of chlorine elimination from Pt-Sn catalyst on the behavior of hydrocarbon reconstruction in propane dehydrogenation

    2019, Catalysis Today
    Citation Excerpt :

    The data in Table 3 provide the evidence that supports this viewpoint. Introduction of chlorine is an essential approach to acidify the support surface and promote Pt dispersion in many industrial reforming catalysts [22,23]. Sachtler [23] also assumed the formation of a surface complex from a promoter, support, and Cl that could alter the acidic function of the catalyst.

  • Synthesis of middle distillate through low temperature Fischer-Tropsch (LTFT) reaction over mesoporous SDA supported cobalt catalysts using syngas equivalent to coal gasification

    2018, Applied Catalysis A: General
    Citation Excerpt :

    The incorporation of Zr and Mg also promotes the catalytic activity of cobalt phase where Zr promotes the reduction of cobalt oxide phase by suppressing the formation of cobalt aluminate and cobalt silicate phase [56]. It is reported by several researcher that ZrOCl2, used as a Zr precursor for catalyst preparation produces ZrOxCly intermediate which act as a mobile intermediate during calcinations step and this mobile intermediate induces higher metal dispersion with smaller crystallite sizes [31,57,58]. Magnesium also promotes catalyst life by inhibiting deposition of coke on catalyst surface.

  • Roles of phosphorous-modified Al<inf>2</inf>O<inf>3</inf> for an enhanced stability of Co/Al<inf>2</inf>O<inf>3</inf> for CO hydrogenation to hydrocarbons

    2017, Journal of Molecular Catalysis A: Chemical
    Citation Excerpt :

    A broad vibration bands centered at 3500–3700 cm−1 can be responsible for an isolated OH stretching vibration mode and the bands at 1630–1650 cm−1 are attributed to an adsorbed H2O molecules (HOH). The vibration bands at around 1118 and 492 cm−1 can be attributed to a triply degenerate P-O stretching vibration mode and to a triply degenerate OPO bending vibration mode of tetrahedral (PO4)3−, respectively [8–10,29–33,60,61]. With an increase of phosphorous content on the CoPAl catalysts, the peak intensity at around 1118 cm−1 increased significantly, which suggests a formation of aluminum phosphate on the outer surfaces of P-Al2O3 support.

  • Controlled preparation and characterization of Pt-Rh/Al<inf>2</inf>O<inf>3</inf> bimetallic catalysts for reactions in reducing conditions

    2016, Applied Catalysis A: General
    Citation Excerpt :

    To characterize Pt-Rh interactions, temperature programmed reduction studies were carried out on catalysts preoxidized at 673 K. If platinum and rhodium are in strong interaction, the metal oxide that is reduced at the lowest temperature may catalyze the reduction of the other one and permits to decrease its reduction temperature [13–15]. In Fig. 3, TPR profiles of bimetallic catalysts are compared to those of the corresponding chlorinated (for CI and MS bimetallic catalysts) and non-chlorinated (for OG bimetallic catalyst) monometallic catalysts.

  • Influence of ceria loading on the NO<inf>x</inf> storage and reduction performance of model Pt-Ba/Al<inf>2</inf>O<inf>3</inf> NSR catalyst

    2015, Catalysis Today
    Citation Excerpt :

    The H2-TPR profile of 4.5% Ce/Al2O3 sample (free of Pt) has also been included in order to better explain the observed reduction peaks. In line with previous studies [24–27], 1.5%Pt/Al2O3 sample presented two main H2 consumption peaks at 200 °C and 360 °C due to the reduction of platinum oxide (PtO or PtO2) to Pt0. The existence of two well defined reduction peaks is thought to be a consequence of different kinds of platinum oxide species interacting with alumina to different extent.

View all citing articles on Scopus
View full text